The present invention relates to a thermal head driving system for use in multicolor or polychromatic printers or duplex or perfecting printers.
In a thermal transferring recording system, for example, a thermal pulse is applied onto a recording medium on which thermally transferable ink (ink having thermal fluidity or sublimatable ink) has been applied (hereinafter referred to as an "ink donor sheet"), so that fluid or sublimated ink is transferred to a sheet of recording paper thereby recording picture information thereon.
FIG. 1 illustrates the principle of recording by a multicolor or polychromatic printer which employs the recording system as mentioned above. A first ink donor sheet on which, for example, black ink has been applied is fed out of a first supply roll 2P, passes between a first backing-up roller 3P and a first thermal head 4P, and is then reeled up on a first winding-up roll 5P. A second ink donor sheet 1Q on which, for example, red ink has been applied is fed out of a second supply roll 2Q, passed between a second backing-up roller 3Q and a second thermal head 4Q, and is then reeled up on a second winding-up roll 5Q. A sheet of recording paper 6 travels in the direction indicated by an arrow to pass first between the first ink donor sheet 1P and the first backing-up roller 3P and then between the second ink donor sheet 1Q and the second backing-up roller 3Q. Thus, two-color recording, in red and black, is accomplished. Similarly, multicolor recording in three colors or more can be made if further recording sections are additionally provided.
FIG. 2 illustrates the principle of recording by a duplex recording printer. At one side (upper side in FIG. 2) with respect to a traveling path 7 of a sheet of recording paper 6, a first supply roll 8A is provided for recording on a first surface of the recording paper 6. An ink donor sheet 8A fed out of the first supply roll 9A passes by the respective under surfaces of a first backing-up roller 3P and a second thermal head 4Q and is then reeled up on a winding-up roll 13A. At the other side (lower side in the drawing) with respect to the traveling path 7, a second supply roll 8B is provided for recording on a second surface of the recording paper 6. A second ink donor sheet 9B fed out of the second supply roll 8B passes by the respective upper surfaces of a first thermal head 4P and a second backing-up roller 3Q which are provided respectively oppositely to the first backing-up roller 3P and the second thermal head 4Q, and is then reeled up on a second winding-up roll 13B.
In this printer, if the recording paper 6 is moved in the direction indicated by an arrow, picture information is first recorded onto the lower surface of the recording paper by the first thermal head 4P and then recorded onto the upper surface of the same by the second thermal head 4Q.
Thus, a plurality of thermal heads are simultaneously driven to record picture information in a multicolor or polychromatic printer or in a duplex or perfecting printer. FIG. 3 shows a conventional thermal head driving system for use in such printers. The respective resistors of thermal heads may result in a variance of their respective resistance values. If thermal heads have different resistance values, the respective amount of heat generated in the thermal heads differ one from another. This results in a poor recorded picture having portions which differ in density one from another depending on the resistance values of the thermal heads. Conventionally, therefore, a number of power sources equal to the number of thermal heads have been provided to separately adjust the amount of heat generated in the individual thermal heads.
Thus, in the two examples as mentioned above, two power sources 15 and 16 have been provided corresponding to the thermal heads 4P and 4Q, as shown in FIG. 3. The respective output voltages of these power sources 15 and 16 are adjusted to be V.sub.P and V.sub.Q in accordance with the resistance values of the thermal heads 4P and 4Q. Assuming that, for example, the respective resistance values of the first and second thermal heads 4P and 4Q are 320.OMEGA. and 360.OMEGA., the output voltages V.sub.P and V.sub.Q are adjusted, for example, to be 21 volts and 22 volts, respectively. Thus, the first power source supplies its adjusted output voltage V.sub.P to a first driver 17 to drive the first thermal head 4P in accordance with a first picture signal 18. Similarly thereto, the second power source 18 supplies its adjusted output voltage V.sub.Q to a second driver 19 to drive the second thermal head 4Q in accordance with a second picture signal 21.
Thus, in a thermal head driving system as mentioned above, it has been necessary to provide a number of power sources of the same performance equal to the number of thermal heads, resulting in poor economy and duplication of equipment.